Propylene impact copolymers are commonly used in a variety of applications where strength and impact resistance are desired, such as molded and extruded parts (e.g. automobile parts, household appliances, luggage and furniture). Propylene homopolymers are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
A typical propylene impact copolymer contains two phases or components, a homopolymer component and a copolymer component. These two components are usually produced in a sequential polymerization process wherein the homopolymer produced in a first reactor is transferred to a second reactor where copolymer is produced and incorporated within the matrix of the homopolymer component. The copolymer component has rubbery characteristics and provides the desired impact resistance, whereas the homopolymer component provides overall stiffness.
Many process variables influence the resulting impact copolymer and these have been studied and manipulated to obtain various effects. For example, U.S. Pat. No. 5,166,268 describes a “cold forming” process for producing propylene impact copolymers where finished articles are fabricated at temperatures below the melting point of the preformed material, in this case, the propylene impact copolymer. The described process uses a propylene impact copolymer comprised of either a homopolymer or crystalline copolymer matrix (first component) and at least ten percent by weight of an “interpolymer” of ethylene and a small amount of propylene (the second component). Adding comonomer to the first component is described as reducing its stiffness. The ethylene/propylene copolymer second component is reported to assist the finished, cold-formed article in better maintaining its shape.
U.S. Pat. No. 5,258,464 describes propylene impact copolymers with improved resistance to “stress whitening.” Stress whitening refers to the appearance of white spots at points of impact or other stress. These otherwise conventional propylene impact copolymers have first and second components characterized by a numerical ratio of the second component intrinsic viscosity to the first component intrinsic viscosity which is near unity.
In U.S. Pat. No. 5,362,782, nucleating agent is added to propylene impact copolymers having a numerical ratio of the intrinsic viscosity of the copolymer rubber phase (second component) to the intrinsic viscosity of the homopolymer phase (first component) which is near unity, and an ethylene content of the copolymer phase in the range of 38% to 60% by weight. These propylene, impact copolymers are described as producing articles having good clarity as well as impact strength and resistance to stress whitening. The nucleating agents are reported to increase stiffness and impact strength.
U.S. Pat. No. 5,250,631 describes a propylene impact copolymer having a homopolypropylene first component and an ethylene/butene/propylene terpolymer second component. Again, the goal is to obtain high impact strength coupled with resistance to stress whitening.
Propylene impact copolymers are also used to produce films as described in U.S. Pat. No. 5,948,839. The impact copolymer described in this patent contains a conventional first component and 25 to 45 weight percent ethylene/propylene second component having from 55 to 65 weight percent ethylene. This impact copolymer composition has a melt flow of from 7 to 60 dg/min. Such films are used in articles such as diapers.
More recently, efforts have been made to prepare propylene impact copolymers using newly developed metallocene catalysis technology in order to capitalize on the benefits such catalysts provide. Homopolymers prepared with such “single-site” catalysts often have narrow molecular weight distributions, and low extractables and a variety of other favorable properties associated therewith. Metallocene catalyzed copolymers typically have narrow composition distributions in addition to narrow molecular weight distribution and low extractables.
Unfortunately, known metallocenes are not able to provide copolymer components with high enough molecular weight under commercially relevant process conditions. The resulting propylene impact copolymers tend to have poor impact strength compared to their conventionally catalyzed counterparts.
U.S. Pat. No. 5,990,242 approaches this problem by using an ethylene/butene (or higher alpha-olefin) copolymer second component, rather than a propylene copolymer, prepared using a hafnocene type metallocene. Such hafnium metallocenes are generally useful for producing relatively higher molecular weight polymers, however, their activities are much lower than the more commonly used zirconocenes. In any event, the second component molecular weights and intrinsic viscosities are lower than desired for good impact strength.
Other references of interest include US 2011/0034649; US 2011/0081817; WO 2007/071446; U.S. Pat. No. 6,429,250; US 2011/0160373; U.S. Pat. No. 8,557,917; WO 01/32757 and U.S. Pat. No. 6,207,750.
Accordingly, there is need for new catalysts and/or processes that produce polypropylene materials that meet the needs for use in impact resistant applications, such as a good stiffness toughness balance.